Support loading multiple images through a list or comma-separated string

[mheppner]

With the current thunder.images.fromtif() call, either a single image or a glob of images in a directory can be loaded. To load specific images, you have to do something like this:

rdds = [
    thunder.images.fromtif('path1'),
    thunder.images.fromtif('path2'),
]
bigRdd = sc.union(rdds)
data = thunder.images.fromrdd(bigRdd)

It would be better for .fromtif() to support a comma-separated string, like Spark's loading functions do:

paths = '/mnt/img1.tif,/mnt/img2.tif,/mnt/img3.tif'
data = thunder.images.fromtif(paths)

And by using a list:

paths = [
    '/mnt/img1.tif',
    '/mnt/img2.tif',
]
data = thunder.iamges.fromtif(paths)